As the demand for integrated circuits having ever-smaller device features continues to increase, the need for improved illumination sources used for inspection of these ever-shrinking devices continues to grow. One such illumination source includes a laser-sustained plasma source. Laser-sustained plasma light sources are capable of producing high-power broadband light. Laser-sustained light sources operate by focusing laser radiation into a gas volume in order to excite the gas, such as argon or xenon, into a plasma state, which is capable of emitting light. This effect is typically referred to as “pumping” the plasma.
The shape of a given plasma is, in part, defined by the laser light intensity distribution near the laser focus. Optical aberrations may impact the quality of laser focus and generally limit the predictability of the resulting plasma shape. In a general sense, any intervening medium may produce optical aberrations in the laser light focused into the bulb of the plasma cell. For example, imperfections in any of the optical elements of an optical pathway used to generate the plasma may produce aberrations in the light. In order to contain the gas used to generate the plasma, an implementing plasma cell requires a “bulb,” which is configured to contain the gas species as well as the generated plasma. The bulb of the plasma cell itself serves as a major source of optical aberration in a laser-sustained plasma light source. The production of optical aberrations in laser-sustained plasma light sources results in larger than desired plasma volumes as well as a lack of an ability to control the bulb envelope. This lack of control in turn leads to irreproducible plasma shapes.
Commonly bulb aberrations are controlled by manufacturing bulbs with minimized bulb aberration. FIGS. 1A through 1C illustrate a series of plasma shapes generated utilizing different bulb shapes, with varying levels of optical aberration. FIG. 1A depicts a plasma shape generated by a “football” shaped bulb, while FIG. 1B depicts a plasma shape generated by a spherical bulb. FIG. 1C illustrates the smallest and brightest of the plasmas. The plasma in FIG. 1C was generated using a cylindrical shaped bulb and possessed the smallest amount of aberrations of the FIGS. 1A-1C. Relying on the minimization of aberrations based on medium selection is limited. In a practical sense, aberrations cannot be entirely avoided. Further, the selection of an aberration minimizing bulb provides no mitigation to aberrations generated by additional elements of the plasma light source or aberrations caused by convection within the plasma light source. Therefore, it would be desirable to provide a system and method for correcting aberrations in a laser-sustained plasma light source that corrects the deficiencies identified in the prior art.